1. Field of the Invention
This invention relates to a hard alloy containing molybdenum and more particularly, it is concerned with a composition of a hard alloy comprising a hard phase consisting of at least one compound having a crystal structure of simple hexagonal MC type (M: metal, C: carbon) and a binder phase, which alloy is suitable for use in making tools capable of resisting high impact for a long time.
2. Description of the Prior Art
The first report on a (Mo, W)C base alloy is British Pat. No. 635,221. This describes a process for producing the (Mo, W)C base alloy by nitriding oxides of molybdenum and tungsten in an ammonia stream, carburizing the nitrides with release of nitrogen, adding an auxiliary metal in powder form to serve as a binder in the sintered alloy, and sintering. This alloy was new at that time as an alloy consisting of one or two carbides of (W, Mo)C and (W, Mo).sub.2 C with a binder metal, but has not been put to practical use.
Molybdenum monocarbide (MoC) is considered a useful substitute, since this carbide has the same crystal structure, a simple hexagonal type, as tungsten carbide as well as mechanical properties similar to tungsten carbide. However, the existence of the hexagonal molybdenum monocarbide as a simple substance has remained questionable, and thus an attempt to stabilize molybdenum has exclusively been carried out by forming a solid solution with tungsten carbide. This method was firstly reported by W. Dawihl in 1950, but this solid solution was not examined in detail and its commercial worth was not apparent in those days.
Of late, however, the study to utilize the solid solution (Mo.sub.x W.sub.y)C where x+y=1 has become active with the rise of the price of tungsten. It is very interesting why a study on this solid solution and an attempt to use the same has not been carried out so actively up to the present time.
In the prior art process for the production of a solid solution of MoC-WC, WC, Mo and C powders or W, Mo, C and Co powders are mixed, charged in a carbon vessel and reacted at a temperature of 1660.degree. to 2000.degree. C. (W. Dawhil: "Zeitschrift f. Anorganische Chemie" 262 (1950) 212). In this case, cobalt serves to aid in forming the carbide and to dissolve molybdenum and carbon in the tungsten carbide. In the absence of cobalt, it is very difficult to obtain a solid solution of (Mo, W)C. When a (Mo, W)C powder obtained by this method is used for the production of a cemented carbide alloy with a binder metal of cobalt as a substitute for WC, however, (Mo, W)C decomposes in the alloy to deposit needle crystals of (Mo, W).sub.2 C. Deposition of even a small amount of such a subcarbide with aggregation in the alloy causes deterioration of the strength of the alloy. For this reason, the use of MoC as a substitute for WC has not been attempted with positive results.
In a process for the production of mixed carbides, in general, carbides are heated in the presence of each other, optionally using a diffusion aid such as cobalt, to give a uniform solid solution in most cases, but in the case of a composition of solid solution containing at least 70% of MoC, a uniform solid solution cannot be obtained by counter diffusion only at a high temperature. This is due to the fact that MoC is unstable at a high temperature and is decomposed into solid solutions such as (Mo, W)C.sub.1-x and (Mo, W).sub.3 C.sub.2 and, consequently, a solid solution (Mo, W)C of WC type cannot be formed only by cooling it. As a method of stabilizing this carbide, it has been proposed to react the components once at a high temperature, to effect diffusion of Mo.sub.2 C and WC, and to hold the product at a low temperature for a long time (Japanese Patent Application (OPI) No. 146306/1976). However, a considerably long diffusion time and long recrystallization time are required for forming (Mo, W)C from (Mo, W)C.sub.1-x and (Mo, W).sub.3 C.sub.2 at a low temperature. For the practice of this method on a commercial scale, the mixture should be heated for a long time in a furnace to obtain a complete carbide. This means that the productivity per furnace is lowered and a number of furnaces are thus required. When using a continuous furnace, on the other hand, a long furnace is necessary and mass production is difficult on an industrial scale.
Under these circumstances, the inventors have made various efforts to provide a solid solution (Mo-W) in an economical manner based on the thought that if an alloy consisting of a solid solution (Mo-W) can be prepared at a low cost and a (Mo-W)C powder as a hard material can readily be produced on a commercial scale, the use of these materials or their cemented carbide alloys will remarkably be increased. Consequently, the inventors have reached an invention as disclosed in U.S. Pat. No. 4,216,009 which consists in a process for the production of an alloy powder containing molybdenum and tungsten and having a crystal structure of simple hexagonal WC type, comprising mixing molybdenum and tungsten in the form of compounds thereof selected from the group consisting of oxides, hydroxides, chlorides, sulfates, nitrates, metallic acids, salts of metallic acids and mixtures thereof, the resulting mixture of the compounds having a particle size of at most 1 micron, reducing the mixture with at least one member selected from the group consisting of hydrogen and ammonia to form an alloy powder of molybdenum and tungsten, and then carburizing the alloy powder.
Furthermore, the inventors have proposed cemented carbide alloys as disclosed in Japanese Patent Application (OPI) Nos. 145,146/1980 and 148,742/1980, which are suitable for impact resisting tools. The former invention provides an impact resisting cemented carbide alloy containing molybdenum, characterised in that the friction coefficient is less than 70% of that between WC-Co type alloys and steels, but this alloy does not have a sufficient life, in particular, when subjected to repeated impacts, because it contains a hard phase of the MC type. The latter invention provides an impact resisting cemented carbide alloy comprising a hard phase of mixed carbides of molybdenum and tungsten of the MC type and a binder phase of cobalt and nickel, represented by (Mo.sub.x W.sub.1-x)C.sub.z -(Ni.sub.y Co.sub.1-y) where 0.5.ltoreq.x.ltoreq.0.95, 0.5.ltoreq.y.ltoreq.1.0 and 0.90.ltoreq.z.ltoreq.0.98, but this alloy does not have a long life under such severe condition as encountered when subjected to high impacts for a long time.